Some modern mobile devices contain a variety of sensors that provide useful dynamic information about the current state of the device. For example, some devices include a three axis accelerometer that measures linear acceleration within a local inertial frame and a three axis magnetometer. One purpose of the accelerometer and magnetometer is to provide device orientation measurements (e.g., pitch, roll and yaw angles). Some devices include a Global Position System (GPS) receiver that provides three dimensional location measurements in real-time for use by applications (e.g., map services). In some mobile devices, other sensors are also included, such as a proximity sensor to detect the mobile device's proximity to a user's ear, and image or audio sensors for capturing images, voice, and/or video. The data collected from the sensors on a mobile device and other contextual data associated with the mobile device can be used for various applications.